build: don't store timestamps in APK packages

build: don't store timestamps in APK packages

This commit statically sets SOURCE_DATE_EPOCH to 0, which causes `apk mkpkg` to
drop the date from generated packages. This improves reproducibility while
leaving package specific SOURCE_DATE_EPOCH untouched. The latter can be
interesting for i.e. packages like busybox, which prominently show that date on
the login shell.

Without this commit, packages that live inside feed repositories (openwrt.git,
packages.git etc) like `mtd` or `attendedsysupgrade-common` would have random
timestamps since no "full" git log was available.

Below an example with different source dates (0, 1):

│  0755  root:wheel    -                           -  /
│  0755  root:wheel    -                           -  lib/
│  0755  root:wheel    -                           -  lib/apk/
│  0755  root:wheel    -                           -  lib/apk/packages/
│ -0644  root:wheel    1970-01-01T00:00:01Z       10  lib/apk/packages/mtd.list
│ +0644  root:wheel    -                          10  lib/apk/packages/mtd.list
│  0755  root:wheel    -                           -  sbin/
│ -0755  root:wheel    1970-01-01T00:00:01Z    24595  sbin/mtd
│ +0755  root:wheel    -                       24595  sbin/mtd

Link: https://github.com/openwrt/openwrt/pull/24005
Signed-off-by: Paul Spooren <mail@aparcar.org>